Classification of Living Things 20-3 Chapter 20

Classification of Living Things 2 Cladistic Systematics Now that we know how to read phylogenetic trees….how are they made? Based on shared anatomical characteristics Cladogram: a branching diagram that shows the relationship among species in regard to their shared derived characteristics A cladogram is a special type of phylogenetic tree

Classification of Living Things 3 Cladistic Systematics Cladogram: Traces evolutionary history of the group under study Uses shared derived characters to:  Classify organisms, and  Arrange taxa into a cladogram ­A clade is an evolutionary branch that includes:  A common ancestor, together with  All its descendent species

Constructing a Cladogram When constructing a cladogram, the first step is to make a chart with the characteristics to be compared. Notice in the chart on the right all of the organisms have a notochord, but not all of them are vertebrates. 4

Constructing a Cladogram Only the newt, snake and lizard have lungs and a 3-chambered heart. Snakes and lizards have internalization fertilization and amniotic membranes in their eggs. Reptiles (snakes and lizards) are a clade Clade: an evolutionary branch that includes a common ancestor together with all its descendant species 5

Constructing a Cladogram The newt (an amphibian) and the eel (fish) are all evolutionary descendants of reptiles The next step is to draw a cladogram showing these relationships The goal is to have the simplest drawing possible What is the deal then with bony limbs and long cylindrical body? 6

Constructing a Cladogram parsimony: parsimony: the fewest number of assumptions is the most logical. results in the simplest cladogram possible 7

Classification of Living Things 8Parsimony Cladists are always guided by the principle of parsimony  The arrangement requiring the fewest assumptions is preferred  This would: ­Leave the fewest number of shared derived characters unexplained ­Minimize the number of assumed evolutionary changes The reliability of a cladogram is dependent on the knowledge and skill of the investigator

Alternate, Simplified Cladograms a. a.X, Y and Z share the same characteristics. b. & c. b. and c. are unlikely examples as the shared characteristics in Y and Z would have had to evolve independently 9

10 Cladistic Versus Traditional View of Reptilian Phylogeny

Classification of Living Things 11 Phenetic Systematics Assumes it will never be possible to construct a truly phylogenetic classification system Species are classified according to the total number of shared similarities  Disregards assumed phylogenetic considerations  Ignores issues of convergent or parallel evolution

Classification of Living Things 12 Traditional Systematics Mainly uses anatomical data  Classify organisms using assumed phylogeny with emphasis on phenotype  Stress both common ancestry and degree of structural difference among divergent groups  Construct phylogenetic trees by applying evolutionary principles to categories  Not strict in making sure all taxa are monophyletic